Which of these numbers is prime? ${8,\ 19,\ 55,\ 95,\ 99}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 8 are 1, 2, 4, and 8. The factors of 19 are 1 and 19. The factors of 55 are 1, 5, 11, and 55. The factors of 95 are 1, 5, 19, and 95. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 19 is a prime number.